1. Field of the Invention
The present invention relates to laser scanning systems for reading and decoding bar code symbols or other types of symbology. Particularly, it relates to an analog waveform decoder that operates directly upon ana analog waveform representative of a bar code symbol for decoding the symbol. Still more particularly, the invention relates to an analog waveform decoder that can decode high density bar code symbols reliably and efficiently.
2. Discussion of the Prior Art
Many industries, particularly the grocery and food processing industry, have been designating their products with unique bar code symbols. A bar code usually consists of alternating stripes of two colors with the information such as a product identification number, encoded in the widths therein. Various bar code schemes have been developed and today the most widely used codes are the Universal Product Code and Code 39.
Current bar code technology is centered around a laser scanner, a hardware digitizer and decoding software. A bar code is read by analyzing the waveform produced when light from a scanning laser beam is reflected from the bar code image area, i.e., when the bar code is convolved with the point spread function of a light source--the laser. Typically, the hardware digitizer takes as input the resulting analog waveform, and produces as an output a representation of that waveform in binary signal form, i.e., a sequence of numbers describing the widths of the alternating stripes of the bar code After this is accomplished, an effort is made to decode the sequence of widths. This method is simple and works quite well when the width of the point spread function of the laser beam is small compared to the widths of the bar code. However, this system fails for high density bar code symbols, particularly because the phenomenon of convolution distortion affects the analog representation of the bar code symbol that should be, in the ideal case, a series of rectangular pulses. Other types of noise and distortions affecting the analog waveform, and hence the decodability of the bar code, exist These are introduced by paper grain noise, printing noise, dot matrix printing, and low contrast printing of the bar code symbols. All of these affect the resultant analog waveform in different ways and each can prevent the recovery of pulse widths from being a simple matter. For example, convolution distortion essentially causes an averaging out of the ideal signal and can make the pulses, as represented in the analog waveform, appear wider and more rounder than they really are. This can cause significant decoding errors even in the absence of noise.
Another major source of distortion is the ink spread. Ink spread occurs when ink deposited on the paper flows beyond the intended boundaries. Depending upon the way the bar code is printed, ink spread can effectively change the character of the bar or space, and possibly result in a misdecode. Though the sequence of the bars in a bar code are discernable to the human eye, a typical decoder must be able to discriminate differences in widths of the order of 0.01 inches. It should be stressed that the effects of this type of distortion are most significant near the edges between the bars and spaces.
Furthermore, as mentioned above, noise presents decoding problems. Dot-matrix symbol noise can add small peaks and valleys to the analog waveform representative of the bar code symbol. The size of the added peaks and valleys is related to the dot size of the printer. Other types of noise such as low frequency noise due to artificial ambient light, or shot noise due to sunlight and/or the electronics of the scanner can create additive noise and effect the sensitivity of the edges. Since additive noise corrupts the amplitude of the analog waveform, thresholds are used to decide which peaks of the waveform are significant and should be reported as binary pulses. This thresholding limits the working range and spatial resolution of the current bar code readers--especially since high density waveforms resemble noise in low density bar codes and have their waveform peaks/valleys removed or merged by the hardware digitizer.
In view of the above-mentioned limitations and disadvantages of the current bar code decoding technology, an analog waveform decoder that has an extended working range and greater resolution would be desirable. Particularly, an analog decoder which can handle increased convolution distortion and dot-matrix symbol distortion would be extremely advantageous. It is accordingly an object of the present invention to fulfill these needs by providing an analog waveform decoder that obviates the need for a hardware digitizer and operates directly upon the analog waveform. Such analog decoding may be accomplished either by direct decoding of the waveform or by deblurring the waveform to obtain a form close to the ideal input and then decode it by conventional means. In addition, it would be particularly advantageous to provide an analog waveform decoder that can effectively and efficiently decode high density bar code symbols that are in current use and one that can be adapted to decode high density symbology of the future.